1. Field of the Invention
This invention relates to microwave multiplexers for use in satellite communications and, in particular, to multiplexers having two to four channels and containing a plurality of independent dielectric cut resonators where the total number of cut resonators exceeds the number of cavities in the multiplexer.
2. Description of the Prior Art
Previous microwave multiplexing networks have a pluality of bandpass filters suitably arranged on a coaxial manifold. Presently, space-application microwave filters with passbands centered between 1 and 4 GHz have generally employed coaxial resonator (re-entrant) cavities for the realization of the filter resonator elements. These coaxial filters can be operated to produce satisfactory results.
Above 4 GHz, low power waveguide filter cavities may conveniently be replaced by single or dual mode dielectric resonators. This results in very significant mass and volume savings for the filter without degradation to the overall electrical performance. These types of dielectric-loaded structures have been described by S. J. Fiedziuszko (IEEE MTT-30, No. 9, Sept., 1982, pp. 1311-1316) and W. C. Tang, et al. U.S. Pat. No. 4,675,630 entitled Triple Mode Dielectric Loaded Bandpass Filters and U.S. Pat. No. 4,652,843 entitled Planar Dielectric Resonator Dual-Mode Filter). If these dielectric resonators are used to replace coaxial cavities at S-band or L-band, the resulting overall filter may actually end up heavier than the coaxial filters that are replaced. Moreover, the power handling capability will be reduced and the far out-of-band spurious response behaviour will be worse as compared to the coaxial technology. Although the insertion loss of the dielectric filter will be lower, this advantage is outweighted by its drawbacks as far as most space applications are concerned. For these reasons, coaxial cavity filters have generally been used in space applications in the S-band or L-band and have not been replaced by dielectric resonator filters.
It is known to construct high power bandpass filters by cascading single quarter-cut image resonators (see IEEE MTT-S International Microwave Symposium Digest, June 9-11, 1987, pp. 133-136, published by T. Nishikawa, et al. and entitled "Dielectric High-Power Bandpass Filter Using Quarter-Cut TE.sub.01.delta. Image Resonator for Cellular Base Stations"). This structure provides high handling capability and is thus suitable for cellular radio application. However, it is not suitable for space applications. The filter uses a single mode configuration and is larger in mass and volume relative to dielectric resonator filters operating in the dual-mode configuration.